Ferrous powder metal (P/M) parts which are produced by conventional pressing and sintering processes, typically exhibit low impact and fatigue strength due to pores remaining in these parts after sintering. For many years however, these dynamic properties have been improved by infiltrating the sintered parts with copper or a copper based alloy, in an attempt to reach near full density. Although significant improvements in tensile and fatigue strength have been achieved, the improvement in impact strength has until recently been insufficient to permit use as high performance parts, which currently are thus made by more expensive powder forging and hot pressing methods.
Increased tensile and fatigue strengths have been achieved by heat treating an infiltrated part, however this typically results in reduced impact strength. Improvement in tensile and fatigue strength without loss of impact strength (toughness) and ductility would be an important advance toward the acceptance of infiltrated ferrous parts for high performance applications.
Prior to its commercialization in about 1946, copper infiltration of ferrous parts suffered from large positive dimensional changes taking place during infiltration. Of the growth-controlling additives known and used in the pressing and sintering of P/M parts, i.e., phosphorus, boron, carbon, lithium, silver, in the elemental or alloy form, carbon in the form of graphite came to be used exclusively for copper infiltration of ferrous parts. Carbon not only decreased the large positive dimensional changes down to manageable levels but also brought about desirable and clean reduction of oxides. It is for these reasons that today graphite additions corresponding to a combined carbon content (based on the iron content of the copper infiltrated part) from about 0.5% to about 0.8% are most commonly used in the industrial practice of copper infiltration of ferrous parts. At these levels of carbon, overall growth can be kept below about 0.7%.
There is, however, another phenomenon known as distortion that appears to be specific and peculiar to copper infiltrated parts. Distortion refers to the non-uniform, often erratic, dimensional changes taking place during infiltration, which cause dimensional tolerances of copper-infiltrated ferrous parts to be substantially inferior to those obtained by pressing and sintering. P/M parts made by pressing and sintering are often sized to improve dimensional tolerances. Infiltrated ferrous parts, however, do not respond very well to sizing because of their high strength and high density. Distortion is therefore an even more serious problem for copper infiltrated parts and a solution to this problem would enable a wider application of copper infiltration. This phenomenon is distinctly different from dimensional change. The dimensional change of a pressed and sintered part is typically symmetrical in directions parallel and perpendicular to the direction of pressing. In copper infiltration, however, this symmetry is lost and one therefore speaks of swelling and distortion. It appears that kinetic factors and topology and distribution of pores play an important role in this phenomenon. Copper infiltrated parts typically have dimensional tolerances inferior to parts made by pressing and sintering. As a result, the usefulness of such parts is restricted to applications requiring a lesser degree of dimensional accuracy, or, secondary machining may be necessary to improve the dimensional tolerances of infiltrated parts.
As described in U.S. Pat. Nos. 4,606,768 and 4,731,118 which are specifically incorporated by reference herein, the state-of-the-art of copper infiltration of P/M steels has advanced to a point where it is now possible to obtain copper infiltrated parts having Charpy un-notched impact strength and ultimate tensile strength of over 240 ft. lbs. and 96 ksi, respectively, or fatigue endurance limits of 65 ksi. These property values are dramatically superior to those obtained before.
It is not unusual, however, that these dynamic properties, for example impact strength, sometimes show differences of up to 100% within a batch of parts for a significant number of parts. The usefulness of these superior properties can therefore be exploited fully only to the extent that they can be uniformly and predictably distributed within a given batch of parts. This appears to be true regardless of whether the parts are made by the traditional infiltration technology giving impact strengths between about 10 and 20 ft. lbs. or by the advanced high performance infiltration technology giving impact strengths of between 120 to 240 ft. lbs. or more. The uniformity of properties appears to depend on a large number of both raw material and process related factors which are often difficult or impossible to predict in a given situation, this results in a higher incidence of parts which might not meet a desired specification and hence economic waste.